Three Phase Ungrounded Fault - Symmetrical Components

There cannot be a significant ground fault current in an ungrounded system, unsurprisingly. The only faults that can occur are circuit overcurrent, for which the usual suspects of circuit breaker or fuse are applicable, and ground fault, which is expressed by the phase voltage with respect to earth/ground dropping.

• In hospitals, which is where ungrounded systems can be found, a pesky ground fault can be tolerated without disconnecting the supply, on the basis that it is better to sustain a ground fault and report it rather than disconnect a piece of equipment that is supplying precious life-support!

Any classic textbook on Symmetrical Components will answer your question, you will even find out that a three phase fault has exactly the same current as a three phase to ground fault, even more magically you will find that only positive sequence currents flow making it the easiest fault to calculate.

In my opinion, in ungrounded-or High Resistance Grounded-system a phase-to-ground-to-phase will be the highest level current. It is difficult to imagine a three phase-to-ground in the same time. However, only a very small part –negligible- of

this current will return to neutral through grounding resistor.

Through the neutral grounding resistor the highest level current will flow only in

one phase-to-ground fault case.

See[for information]:

Fortunately this is engineering science, not political science, where facts are more important than opinions. From your GE reference, page 12, Three-phase Bolted Short Circuit: "A three-phase bolted short-circuit...generally results in the maximum short-circuit values and for this reason is the basic short-circuit calculation in commercial and industrial power systems."

Granted this is the least likely fault (less than 5% occurrence vs. all other types), but it yields a conservative value; i.e., no other type of fault will exceed this value. (There are certain exceptions to this, but only those engineers that deal with switchgear directly connected to utility-sized generators that are also grid-connected, will ever encounter these specialized situations. That's why segregated phase bus is used between the generator and the step-up transformer, specifically to prevent such faults.)

To continue from the reference (and to address OP's question), "In most three-phase power systems, the levels of line-to-line bolted short circuits currents are approximately 87% of three-phase bolted short circuits currents, but this calculation is seldom required because it is not the maximum value."

Thank you. I agree with you about the difference between politics and science, of course. However, in the actual conditions when no "scientific" data is provided one could only to express his opinions.

That is the disadvantage copying from my Excel Workbook:

See[for information]:

GE Application Information Short-Circuit Current Calculations

http://apps.geindustrial.com/publibrary/checkout/GET-3550F?TNR=White%20Papers|GET-3550F|generic

Post Glover NEUTRAL GROUNDING RESISTORS TECHNICAL

http://www.postglover.com/download/neutral-grounding-resistors-technical-information/

I think it could be better